This invention relates generally to the field of disc drive data storage devices or disc drives, and more particularly, but not by way of limitation, to an improved glide test head for testing for the presence of asperities on the surface of magnetic data recording discs.
Disc drives of the type known as "Winchester" disc drives or hard disc drives are well known in the industry. Such disc drives record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent to the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator housing opposite to the coil, the actuator housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator housing rotates, the heads are moved radially across the data tracks along an arcuate path.
As the physical size of disc drives has decreased historically, the physical size of many of the disc drive components has also decreased to accommodate this size reduction. Similarly, the density of the data recorded on the magnetic media has been greatly increased. In order to accomplish this increase in data density, significant improvements in both the recording heads and recording media have been made.
For instance, the first rigid disc drives used in personal computers had a data capacity of only 10 megabytes, and were in the format commonly referred to in the industry as the "full height, 51/4" format. Disc drives of the current generation typically have a data capacity of over a gigabyte (and frequently several gigabytes) in a 31/2" package which is only one fourth the size of the full height, 51/4" format or less. Even smaller standard physical disc drive package formats, such as 21/2" and 1.8", have been established. In order for these smaller envelope standards to gain market acceptance, even greater recording densities must be achieved.
The recording heads used in disc drives have evolved from monolithic inductive heads to composite inductive heads (without and with metal-in-gap technology) to thin-film heads fabricated using semi-conductor deposition techniques to the current generation of thin-film heads incorporating inductive write and magneto-resistive (MR) read elements. This technology path was necessitated by the need to continuously reduce the size of the gap in the head used to record and recover data, since such a gap size reduction was needed to reduce the size of the individual bit domain and allow greater recording density.
Since the reduction in gap size also meant that the head had to be closer to the recording medium, the quest for increased data density also lead to a parallel evolution in the technology of the recording medium. The earliest Winchester disc drives included discs coated with "particulate" recording layers. That is, small particles of ferrous oxide were suspended in a non-magnetic adhesive and applied to the disc substrate. With such discs, the size of the magnetic domain required to record a flux transition was clearly limited by the average size of the oxide particles and how closely these oxide particles were spaced within the adhesive matrix. The smoothness and flatness of the disc surface was also similarly limited. However, since the size of contemporary head gaps allowed data recording and retrieval with a head flying height of twelve microinches (0.000012 inches, 12.mu.") or greater, the surface characteristics of the discs were adequate for the times.
Disc drives of the current generation incorporate heads that fly at nominal heights of only about 2.0.mu.", and products currently under development will reduce this flying height to 1.5.mu." or less. Obviously, with nominal flying heights in this range, the surface characteristics of the disc medium must be much more closely controlled than was the case only a short time ago.
In current disc drive manufacturing environments, it is common to subject each disc to component level testing before it is assembled into a disc drive. One type of disc test is referred to as a "glide" test, which is used as a go/no-go test for surface defects or asperities, or excessive surface roughness. A glide test typically employs precision spin stand and a pzt-glide test head. The glide test is performed with the test head flown at approximately half the flying height at which the operational read/write head will fly in the finished disc drive product. For instance, if the disc being glide tested is intended for inclusion in a disc drive in which the operational heads will fly at 2.0.mu.", the glide test will typically be performed with the glide test head flying at 1.0.mu.". If the glide test is completed without contact between the glide test head and any surface defects, then the disc is passed on the assumption that there will be no contact between the operational heads and the discs during normal operation with a nominal head flying height twice that of the glide test head flying height.
A typical glide test head includes a piezoelectric crystal, or piezo element, mounted on a wing that extends laterally from the side of the glide test head. Any contact between the air bearing surfaces of the glide test head and a surface asperity on the disc results in excitation or ringing of the slider body of the glide test head. This excitation of the slider is passed to the piezo element which outputs a small electrical signal on attached wires. The electrical signal is detected by associated electronic circuitry, which commonly is connected as an input to go/no-go programming. Thus, if the air bearing surfaces are contacted at any point by a media surface defect or asperity during the glide test, the disc is scrapped as unusable in the disc drive.
The output from the piezo element of the glide test head is a function, among other parameters, of the location of contact between the glide test head and the disc surface asperity. However, glide test heads of the current generation provide no direct information related to the specific location of the slider/asperity contact location relative to the piezo element.
In monitoring the overall quality of disc media, it would be useful to not only know that contact between a glide test head and a surface asperity has occurred, but also to know the precise location of such contact. With this information, further analysis of the nature of the disc surface asperity can be undertaken, such as a microscopic examination of the asperity location. This type of detailed analysis can be useful for monitoring the quality of disc media from numerous sources, and providing information back to the disc supplier for use in improving his manufacturing and handling processes.
A need clearly exists, therefore, for a glide test head which can more precisely define the location of surface asperities on disc media surfaces.